This invention generally relates to optical fiber transmission systems and is more particularly concerned with a device for stabilizing the optical output power of the transmitter module in a data transmission system, especially a system having a high bit stream rate of the order of 140 Mbits/s, for example.
It can be stated in general terms that, in an optical fiber transmission system, the transmitter module comprises a transmitter base consisting of a laser diode, a control photodiode and a laser-fiber optical coupling system inserted in a plug for connecting said base to the optical fiber cable, and a control electronic system for performing, among other functions, the regulation or stabilization of the optical power output.
It is also known that, in the presence of data to be transmitted, the current of the laser diode employed in the transmitter module results from superposition of its d.c. bias current and its modulation current, with the result that the amplitude of the light pulses transmitted through the optical fiber depends on the optical output power produced by the laser diode. Since it is desired to transmit the light pulses at the maximum output power of the laser diode, it proves necessary to stabilize both the maximum optical power and the minimum optical power in order to maintain the amplitude of the optical output power of the laser diode at a constant value. This stabilization is obtained by regulating the modulation current of the laser diode. Furthermore, when a temperature variation of the casing which contains the laser diode takes place, the transfer characteristic of the laser diode (output power as a function of current intensity) is modified, with the result that a temperature-dependent regulation of the laser diode d.c. bias current permits detection of aging of the laser diode which is identical within the range of operating temperatures.
In the case of a high bit-rate data transmission system, it is already possible by means of a known device to stabilize the optical power output of the laser diode employed in the transmitter module. A device of this type is described in the article published in "I.E.E.E. Transactions on Communications", vol. 26, No. 7, July 1978, pages 1088 to 1097 and entitled "Electronic Circuits for High Bit Rate Digital Fiber Optic Communication Systems". In FIG. 8 of this article, the device for stabilizing the optical output power produced by the laser diode consists of a first control loop comprising a modulating-signal peak-to-peak amplitude detector in series with a differentiator for stabilizing the maximum optical output power and the minimum optical output power produced by the laser diode, thus maintaining the amplitude of modulation of the optical output power at a constant value, and a second control loop comprising a d.c. amplifier in series with a differentiator for stabilizing the laser diode d.c. bias voltage corresponding to the minimum optical output power produced by said diode.
However, a device of this type for stabilizing the optical output power of a laser diode is subject to disadvantages. In fact, when digital data are not present, it is necessary to put both control loops out of service at the same time. Furthermore, by reason of stabilization of the laser diode bias voltage at a value corresponding to the minimum optical output power produced by said diode, said device is capable of transmitting neither analog data nor data in multilevel codes. Moreover, the structure of this device is relatively complex.